We will use x-ray crystallographic methods to determine the molecular structure of a penicillin receptor, the D-alanyl-carboxypeptidase-transpeptidase from streptomyces R61. Examination of its interaction with a series of Beta-lactam antibiotics will allow us to correlate their relative activity with their binding geometry to the receptor. Coupled with our concurrent X-ray analysis of a penicillin-destroying Beta-lactamase enzyme, this study could conceivably lead to new Beta-lactams which are not only more effective inhibitors of bacterial cell wall receptors, but also more resistant to hydrolysis by the medically-troublesome Beta-lactamases. Preliminary crystallographic data have been reported (Knox et al. J. Molec. Biol. 127, 217-218 (1979)).